The present invention relates generally to a system and method for satellite positioning systems, and more particularly to a system and method for providing time to a satellite positioning system receiver to assist it in satellite acquisition.
Generally, satellite positioning systems (SPS), (for example, Navstar or Global Positioning System (GPS) is a widely used SPS system), provide an invaluable service that has perhaps exceeded the imagination of the designers of the systems. For example, SPS systems are used in military applications (such as providing targeting information for smart bombs, navigation guidance for vehicles and foot soldiers, etc.), commercial applications (such as tracking delivery trucks, measuring the performance of vehicles, providing position location to a person on foot or in a vehicle, etc.), and medical and scientific applications (such as assisting in the location of persons in need of assistance, tracking animal migrations, etc.).
While SPS systems have become widely used today, there remains a major problem that hinders their use in certain situations. Since the satellites are in high-earth orbit (for example, the satellites in the GPS system orbit the Earth at approximately 11,000 nautical miles), the signals that are transmitted by the satellites are usually very weak by the time they reach a SPS receiver. Because the satellites must provide their own power (typically through the use of solar panels), the transmit signal strength cannot simply be increased, since doing so may consume more power than the solar panels are able to provide. Due to the relatively low transmit signal power, SPS signals are attenuated by thick foliage, buildings, tunnels, etc. to a point where the SPS signals fall below a minimum signal power threshold and becomes difficult (if not impossible) for the SPS receiver to detect.
When a SPS receiver is first turned on, it must be able to receive the transmitted signals from a certain number of SPS satellites (for example, in the GPS system, the GPS receiver should be able to receive the transmitted signal from at least three or four GPS satellites) before it can determine its position and the current time. Each satellite in the SPS system transmits a unique signal that can be used by the SPS receiver (in conjunction with signals from other SPS satellites) to calculate the SPS receiver""s position and time. One of the most vital pieces of information that is transmitted in the SPS signal is a highly accurate timing signal. Along with the timing signal, other information (commonly referred to as ephemeris data) is transmitted by a SPS satellite. The timing signals transmitted by the SPS satellites are usually referenced to a particular time, for example, Greenwich Mean Time. The differences between the various timing signals received by the SPS receiver and its own internal clock are then used to calculate the position of the SPS receiver and the current time.
The low transmit signal power, when compounded with the natural attenuation (signal strength being inversely proportional to distance) of the transmit signals, may prevent SPS receivers from being able to acquire the signal that is transmitted from the SPS satellites (or from a sufficient number of SPS satellites). A low transmit signal power that is further attenuated by objects in the operating environment of a SPS receiver can slow down the acquisition process to the order of several minutes (or more) or it may prevent the acquisition from occurring altogether.
It has been noted that if a SPS receiver can be provided with an accurate timing reference (along with ephemeris data) through a source other than the actual signals transmitted by the SPS satellites, the acquisition process can be accelerated (or completed if it was previously unable to do so). Moreover, the more accurate the timing reference that is provided through an alternate source, the easier the acquisition process becomes.
A widely available communications network that can itself be used to provide an accurate timing reference (along with ephemeris data) is the cellular telephone network. Certain types of cellular telephone networks have highly accurate built-in clocks. For cellular telephone networks without built-in clocks, such clocks may be added at the cell-sites (base stations). One proposed solution for helping the SPS receiver achieve faster satellite acquisition makes use of an enhanced SPS receiver with a built-in cellular telephone network interface. The enhanced SPS receiver can then communicate with the cellular telephone network and obtain an accurate timing reference from a clock built into the cellular telephone network.
One disadvantage of the prior art is that although there exists a large networks of cellular telephone systems, only a relatively small number of them are equipped to provide the accurate timing reference. For the majority of the remaining cellular telephone systems, an accurate timing reference needs to be installed at high cost.
A second disadvantage of the prior art is that while large networks of cellular telephone systems exist, there are large parts of the world (rural and under-developed regions) where there is no cellular telephone coverage. Therefore, in areas with no cellular telephone coverage, the SPS receivers may still trouble acquiring the signals transmitted by the SPS satellites.
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide a system and method for assisting a satellite positioning system (SPS) receiver in acquiring the transmitted signal from SPS satellites by providing the SPS receiver with accurate timing information via the use of a communications link (preferably wireless) and readily available time servers. Preferred embodiments are particularly effective when signals from the SPS satellites are attenuated.
In accordance with a preferred embodiment of the present invention, a method for providing time information comprising sending a query to a lime server via a first communications network, determining a time-of-day from a response from the time server, adjusting a clock if the time-of-day differs from a time maintained by the clock, and providing the time information to a satellite positioning system (SPS) receiver via a first communications link, wherein the communications link is different from a second communications link that is used by satellites in the SPS to transmit information to the SPS receiver.
In accordance with another preferred embodiment of the present invention, a method for facilitating signal acquisition in a satellite positioning system (SPS) comprising sending a periodic query with a first period to a time server via a communications network, determining a time-of-day from a response from the time server, adjusting a clock if the time-of-day differs from a time maintained by the clock, and providing a time information based on the time maintained in the clock periodically to a SPS receiver with periodicity equal to a second period via a first communications link, wherein the first communications link is different from a second communications link that is used by satellites in the SPS to transmit information to the SPS receiver.
In accordance with another preferred embodiment of the present invention, a system for aiding signal acquisition in a satellite positioning system (SPS) comprising a SPS receiver, the SPS receiver containing circuitry to receive transmissions from a plurality of SPS satellites and compute its current position and a current time from the received transmissions, a communications processor coupled to the SPS receiver, the communications processor containing circuitry to obtain time information from an external source and to provide the time information to the SPS receiver, a first communications network coupled to the communications processor, and a time server coupled to the first communications network, the time server containing circuitry to maintain accurate time and to response to time queries.
In accordance with another preferred embodiment of the present invention, a position measuring device comprising a satellite positioning system (SPS) receiver comprising an analog processing unit containing circuitry to filter and amplify an analog SPS signal received via an antenna, a digital processing unit coupled to the analog processing unit, the digital processing unit containing circuitry to perform signal acquisition, correlation, and decode, a communications processor comprising a pulse and message generator (PMG) unit containing circuitry to produce a timing beacon and a timing message, a real-time clock coupled to the PMG unit, the real-time clock containing circuitry to keep track of time and to generate signals to indicate timing events, and a network client coupled to the real-time clock, the network client containing circuitry to receive timing information from a time server coupled to the communications processor via a network and to provide time adjustments to the real-time clock.
In accordance with another preferred embodiment of the present invention, a communications processor comprising a pulse and message generator (PMG) containing circuitry to produce a timing beacon and a timing message for use by a satellite positioning system receiver, a real-time clock coupled to the PMG, the real-time clock containing circuitry to track time and to generate signals to indicate timing events to the PMG, and a network client coupled to the real-time clock, the network client containing circuitry to receive timing information from a time server coupled to the communications processor via a network and to provide time adjustments to the real-time clock.
An advantage of a preferred embodiment of the present invention is that a large variety of communications links, preferably wireless, can be used to provide the needed accurate timing information that will help the SPS receiver acquire the signal transmitted by the SPS satellites. It generally does not require the use of a specific type of communications link (such as a particular type of cellular telephone network).
A further advantage of a preferred embodiment of the present invention is that the present invention makes use of an existing infrastructure of accurate time keeping hardware, rather than requiring the installation of any of these expensive pieces of equipment.
Yet another advantage of a preferred embodiment of the present invention is that due to the present invention""s flexibility, it can be operable in a larger set of conditions and environments.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basic for modifying or designing other structures or processes for carrying out the same purpose of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.